Opportunistic enteric pathogens are frequently associated with the morbidity of AIDS. Toxoplasma gondii, the major cause of CNS infection in those with AIDS is acquired via oral ingestion and gains entry into the host through the mucosa of the intestinal tract. The intestinal immune environment is unique as it provides a barrier against all potentially invading pathogens including those that are constitutive to the gut flora and must maintain physiological homeostasis despite the tremendous microbial antigenic load. We have observed that oral infection with T. gondii can result in the development of an acute lethal ileitis in the sensitive C57BU6 mice. In this competing renewal we will evaluate the process by which this parasite induces the lethal inflammatory ileitis by focusing on the interaction between gut derived dendritic cells and the major surface antigen of T. gondii, SAG1. Preliminary data suggests that this antigen is the principal mediator of the lethal inflammatory ileitis in both susceptible and resistant strains of mice. The hypothesis to be tested is that SAG1 can activate a specific subpopulation of dendritic cells within the gut that leads to the strong Thl proinflammatory response within the lamina propria and subsequent death. In the first specific aim, DC will be phenotyped and characterized for expression of specific chemokine receptors (e.g. CCR5) and cytokine production (e.g. IL-12). An in vitro model utilizing the transwell system will allow for the assessment of the interaction among various components of the gut associated lymphoid tissue (GALT). In the second specific aim, we will focus on the SAG1 antigen to stimulate the intestinal DC and initiate the subsequent activation of the lamina propria T cells. The hypothesis is that primed DCs producing IL-12 can stimulate a specific sub-population of CD4 T cells producing a Thl -like pattern of cytokines, which are involved in the development of this parasite induced ileitis. The elicited T cell repertoire to specific SAG1 peptides will be characterized as measured by an ex vivo assay system (immunoscope). We also observed that a pre-sensitization with the purified protein can upon restimulation turn the naturally resistant phenotype of the BALB/c into a sensitive one. In the specific aim 3, the mechanism of this sensitization is explored. This model will provide valuable insight into understanding the mechanism of the host mucosal immunity to enteric acquired pathogens in those with AIDS.